I'm afraid you're just out of luck. Restart the adventure until it appears. When generating the world, apparently there are simply no empty cells left for it to appear.
We'll study the generation settings of this world.
It’s the forest thicket one that is labeled as being dangerous. I mean there are only 3 adventure options available to me so far. I’ve done the easy one with no bosses and then the moderate difficulty one with the ice elf boss and all the snowmen, and now there is this forest thicket one. But no boss to be found.